Current aircraft avionics systems including Terrain Alert and Warning Systems (TAWS) accept flight plan information and use this information in the piloting of aircraft. Flight plan information is provided as a series of waypoints having attributes including latitude, longitude, altitude constraints and waypoint identifiers. A flight plan defined by this information is modeled by the system, and the flight plan model is checked against terrain information to locate any instances in which a potentially unsafe flight altitude exists. In flight, the flight path model is updated using aircraft position so that the path from the aircraft position to the next waypoint is reflected in the model. In the event that a potentially unsafe flight altitude is detected, an alert may be issued to the flight crew who can then take appropriate action.
In order to provide accurate terrain alerting, current avionics systems depend on accurate altimeter settings and altitude sensing by a barometric altimeter or Air Data Computer (ADC) for example. Conventional systems do not take into account errors arising from incorrectly set altimeters, ADC sensor errors or errors due to non standard atmospheric conditions (on cold days). Barometric altimeters are calibrated to indicate true altitude under International Standard Atmosphere (ISA) conditions. Any deviation from ISA conditions may result in an erroneous reading on the altimeter. It is routine to set the altimeter for the local pressure when an aircraft is below the transition level but seldom do flight crews consider the effect of cold temperatures on en-route altitude indication. The local altimeter setting provides the necessary correction for the non-standard pressure in the local area but it does not correct for the effect of non-standard temperatures for altitudes above field elevations. The pilot must make the altitude corrections if the temperature source of the altimeter setting is 0° C. (Celsius) or below.
When the temperature is higher than ISA conditions, the true altitude will be higher than the indicated altitude. On the other hand, when the temperature is lower than ISA conditions, the true altitude is lower than the indicated altitude. The altimeter error is relatively minor down to 0° C. but it can be significant with the minimum altitudes required on some approaches to airports in mountainous areas. Moreover, in the context of TAWS, altimeter error due to temperature can cause false alarms or no alarm when one is needed. An illustration of error due to an incorrect altimeter setting is illustrated in FIGS. 1 and 2.
FIG. 1 depicts a flight profile 20 from a conventional TAWS on a standard day with the aircraft altimeter correctly set. As depicted therein, aircraft or ownship position 22 is depicted consistent with the indicated altimeter altitude (i.e., global positioning system (GPS) scale and Barometric scale). In addition, flight plan 24 from a Flight Management System (FMS) is consistent with FMS vertical constraints (Barometric scale). The ownship position 22 in reference to the FMS flight plan 24 is consistent with vertical deviation indicated on a primary flight display (PFD) of the aircraft (not shown). Terrain 26 is depicted on a scale consistent with terrain database altitudes and ownship position 22 and flight plan 24 are depicted accurately in reference to the terrain 26. Referring now to FIG. 2, depicted therein is a flight profile 30 from a conventional TAWS wherein the altimeter has been set in error 250 feet above the correct setting. TAWS depicts the ownship position 32 at 4250 feet (consistent with the altimeter), and continues to depict the FMS flight plan 34 consistent with the vertical waypoint constraint altitudes. However the separation between the ownship position 32 and the terrain 36 is incorrectly depicted as 250 feet greater than actual.
An illustration of error due to a non-standard, cold temperature day is illustrated by FIGS. 1 and 3. As explained above, FIG. 1 depicts the flight profile 20 from a conventional TAWS on a standard day. As is shown therein, ownship position 22 is depicted consistent with FMS waypoint vertical constraints (barometric) and ownship position 22 in reference to FMS flight plan 24 is consistent with the vertical division indicated on the PFD. The terrain 26 is depicted on a scale consistent with terrain database altitudes and ownship position 22 and flight plan 24 are depicted accurately in reference to terrain. In contrast, FIG. 3 depicts a flight profile 40 from a conventional TAWS system on a non-standard day. As is immediately evident, the flight profile 44 is different as compared to the profile 24 in FIG. 3. In particular, the ownship position 42 and flight plan 44 are not depicted accurately in reference to the terrain 46. This figure depicts the case where the non-standard condition (cold temperature) results in the aircraft's true altitude being 250 feet lower than the indicated altitude. The TAWS depicts the ownship position 42 at 4000 feet (consistent with the altimeter), and continues to depict the FMS flight plan 44 consistent with the vertical waypoint constraint altitudes. However, the separation between the ownship position 42 and terrain 46 is incorrectly depicted as 250 feet greater than the actual separation.
In FIG. 3, if the aircraft is controlled along the flight path 44, the 250 feet error would persist, resulting in a hazardously misleading depiction of the separation between the ownship position 42 and flight plan 44 in reference to the terrain 46. The ownship position 43 associated with the dashed line flight plan 45 represents the actual separation that exists due to non-standard atmospheric conditions.
It is desirable therefore to have an avionics device that takes into account altimeter errors due to sensor error, incorrectly set altimeters and non-standard atmospheric conditions.